1-5 there is no optimal size that will be universal for all the cases. Image size you should choose depending on the block width of the element you plan to use it for. For example, read this post how to choose the size of the product images https://docs.woocommerce.com/document/fixing-blurry-product-images/
Use optimized images, for example you may try https://tinypng.com/ service. Smaller image size in KB – faster page loading.
6. We recommend to use optimized images, how you create them – is your choice.
7) Use the lazy load option for the images in the Theme Options > Speed optimization.
Regards
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